Description

Usage

Arguments

The pathname of the tar file: tilde expansion (see
path.expand) will be performed. Alternatively, a
connection that can be used for binary writes.

files

A character vector of filepaths to be archived:
the default is to archive all files under the current directory.

compression

character string giving the type of compression to
be used (default none). Can be abbreviated.

compression_level

integer: the level of compression. Only used
for the internal method.

tar

character string: the path to the command to be used. If
the command itself contains spaces it needs to be quoted (e.g., by
shQuote) – but argument tar can also contain
flags separated from the command by spaces.

extra_flags

any extra flags for an external tar.

Details

This is either a wrapper for a tar command or uses an
internal implementation in R. The latter is used if tarfile
is a connection or if the argument tar is "internal" or
"" (the ‘factory-fresh’ default). Note that whereas
Unix-alike versions of R set the environment variable TAR, its
value is not the default for this function.

Argument extra_flags is passed to an external tar and
so is platform-dependent. Possibly useful values include -h
(follow symbolic links, also -L on some platforms),
--acls, --exclude-backups, --exclude-vcs (and
similar) and on Windows --force-local (so drives can be
included in filepaths: however, this is the default for the
Rtoolstar). For GNU tar,
--format=ustar forces a more portable format. (The default is
set at compilation and will be shown at the end of the output from
tar --help: for version 1.28 ‘out-of-the-box’ it is
--format=gnu, but the manual says the intention is to change
to --format=pax which GNU incorrectly calls ‘POSIX’ –
it was never part of the POSIX standard for tar and should
not be used.)
For libarchive's bsdtar, --format=ustar is more
portable than the default.

One issue which can cause an external command to fail is a command
line too long for the system shell: as from R 3.5.0 this is worked
around if the external command is detected to be GNU tar or
libarchive tar (aka bsdtar).

Note that files = '.' will usually not work with an external
tar as that would expand the list of files after
tarfile is created. (It does work with the default internal
method.)

Value

The return code from system or 0 for the internal
version, invisibly.

Portability

The ‘tar’ format no longer has an agreed standard!
‘Unix Standard Tar’ was part of POSIX 1003.1:1998 but has been
removed in favour of pax, and in any case many common
implementations diverged from the former standard. Many R platforms
use a version of GNU tar (including Rtools on
Windows), but the behaviour seems to be changed with each version. OS
X >= 10.6 and FreeBSD use bsdttar from the
‘libarchive’ project, and commercial Unixes will have their own
versions.

Known problems arise from

The handling of file paths of more than 100 bytes. These were
unsupported in early versions of tar, and supported in one
way by POSIX tar and in another by GNU tar and
yet another by the POSIX pax command which
recent tar programs often support. The internal
implementation warns on paths of more than 100 bytes,
uses the ‘ustar’ way from the 1998 POSIX
standard which supports up to 256 bytes (depending on the path: in
particular the final component is limited to 100 bytes) if possible,
otherwise the GNU way (which is widely supported, including by
untar).

Most formats do not record the encoding of file paths.

(File) links. tar was developed on an OS that used
hard links, and physical files that were referred to more than once
in the list of files to be included were included only once, the
remaining instances being added as links. Later a means to include
symbolic links was added. The internal implementation supports
symbolic links (on OSes that support them), only. Of course, the
question arises as to how links should be unpacked on OSes that do
not support them: for regular files file copies can be used.

Names of links in the ‘ustar’ format are restricted to 100
bytes. There is an GNU extension for arbitrarily long link names,
but bsdtar ignores it. The internal method uses the
GNU extension, with a warning.

Header fields, in particular the padding to be used when
fields are not full or not used. POSIX did define the correct
behaviour but commonly used implementations did (and still do)
not comply.

File sizes. The ‘ustar’ format is restricted to 8GB
per (uncompressed) file.

For portability, avoid file paths of more than 100 bytes and all links
(especially hard links and symbolic links to directories).

The internal implementation writes only the blocks of 512 bytes
required (including trailing blocks of nuls), unlike GNU tar
which by default pads with nul to a multiple of 20 blocks
(10KB). Implementations which pad differ on whether the block padding
should occur before or after compression (or both): padding was
designed for improved performance on physical tape drives.

The ‘ustar’ format records file modification times to a
resolution of 1 second: on file systems with higher resolution it is
conventional to discard fractional seconds.

Note

For users of macOS (formerly OS X). Apple's HFS and HFS+ file systems
have a legacy concept of ‘resource forks’ dating from classic
Mac OS and rarely used nowadays. Apple's version of tar
stores these as separate files in the tarball with names prefixed by
‘._’, and unpacks such files into resource forks (if possible):
other ways of unpacking (including untar in R) unpack
them as separate files.

When argument tar is set to the command tar on macOS,
environment variable COPYFILE_DISABLE=1 is set, which for the
system version of tar prevents these separate files being
included in the tarball.